1. Field of the Invention
The present invention finds an application in data transmission networks, in which light waves are guided by fibers after they have been modulated to carry data to be transmitted.
2. Description of the Prior Art
Before they are modulated the light waves usually have various predetermined wavelengths referred to as the network wavelengths. The present invention relates to the production of switching matrices which are included in the nodes of such networks and more specifically to the production of selectors used in such matrices. A selector has one or more input ports at which it receives some of said light waves. It also receives a selection instruction in the form of an electrical signal designating at least one of the light waves. It has one or more output ports to which it transmits some of the light waves, as indicated by the instruction.
A selector can be of a type referred to as a spatial selector. It then has a plurality of input ports and selects the light waves according to the input port(s) at which they are received. It can instead be of a type referred to as a wavelength selector. It then selects the light waves according to their wavelength. When a particular wavelength is referred to hereinafter, or it is stated that light waves having a particular wavelength are selected, or that a path is assigned to a wavelength, the implication will be that the wavelength concerned is one of the network wavelengths, that the light waves selected are some of the light waves previously mentioned, and that a path is assigned to those light waves. The light waves selected are not only pure light waves, i.e. unmodulated light waves having a precise network wavelength, but also, and more typically, carrier light waves which result from modulating a corresponding pure light wave with data to be transmitted and which therefore cover a wavelength band including that wavelength. The width of that wavelength band is typically around 0.1 nm for a data bit rate of 10 Gbit/s. Any light wave whose wavelength is in that band or sufficiently close to it would then also be selected. A light wave of this kind is referred to hereinafter as one xe2x80x9cassociatedxe2x80x9d with the network wavelength.
A selector is referred to as a xe2x80x9chybridxe2x80x9d selector hereinafter if it is both a spatial selector and a wavelength selector. A typical selector of this kind transmits to its single output port light waves which are received at one of its input ports and have one of the network wavelengths, that port and that wavelength being those designated by the selection instruction.
In the selectors considered hereinafter a light wave is selected by opening an optical gate. Because that gate is not sensitive either to the input port of the light wave or to its wavelength, it can assure its selection function only because it is situated on a path assigned to that light wave. In the case of a wavelength selector, it is therefore necessary to associate a specific system with the selector to assign various paths to the light waves according to their wavelength. Such a system is referred to hereinafter as a xe2x80x9cwavelength separation systemxe2x80x9d.
First and second hybrid selectors known in the art both include first a spatial selector and then a wavelength selector, in the upstream to downstream direction defined by the direction of propagation of the light waves. Each selector has a plurality of input ports respectively connected to a plurality of output ports by optical gates. An intermediate system connects each output port of the spatial selector to each input port of the wavelength selector. It constitutes a wavelength separation system because, of the light waves that it receives from each of the output ports, it transmits to each of the input ports only the light waves which are associated with a single network wavelength and depend on the input port. An output system connects each of the output ports of the wavelength selector to the output port of the hybrid selector.
The first of the selectors known in the art is described in the article xe2x80x9cDesign and Implementation of a Fully Reconfigurable All-optical Cross-connect for High-capacity Multi-wavelength Transport Networkxe2x80x9d by A. Jourdan et al, IEEE Journal of Lightwave Technology, vol 14 No. 6, p. 1198, June 1996.
It has the disadvantage, where the power of the light waves to be transmitted is concerned, that its intermediate system combines the losses of a combiner receiving the light waves at the output of the spatial selector and those of a multiplexer thereafter performing the wavelength separation.
This is why, in the second hybrid selector known in the art, the intermediate system consists of a router which orients the light waves according to their wavelength as soon as they leave the spatial selector.
The second selector known in the art is described in the article xe2x80x9cA 2.56 Tb/s Throughput Packet/Cell-Based Optical Switch-Fabric Demonstratorxe2x80x9d, S. Araki, S. Takahashi, Y. Maeno, Y. Suemura, A. Tajima, H. Takahashi, K. Matsuda, T. Tamanuki, S. Dohmae, N. Henmi-ECOC""98, 20-24 September, Madrid, Spain. It has the disadvantage that the carrier light waves which it transmits are subject to a high level of noise.
One particular object of the present invention is to provide in a simple and low-cost manner a hybrid selector which combines low internal losses with low output noise.
To this end, it provides a hybrid spatial and wavelength selector for optical switching matrices, the selector including guide means defining paths between input ports receiving light waves having diverse wavelengths and at least one output port and a plurality of gates on the paths each adapted to be opened or closed so that the light waves reaching the output port via the gates which are open are selected according to the input ports which received the light waves and according to the wavelengths, wherein the guide means are situated only between the plurality of optical gates and the output port and include a wavelength separation system assigning some of the paths to the light waves according to their wavelength.
To enable some of the optical gates of the hybrid selectors known in the art to implement the wavelength selection function, the wavelength separation system of those selectors has always been located on the paths taken by the light waves on the upstream side of these gates. The inventors decided that, to assign certain paths to certain wavelengths, a system of the above kind could just as well operate on the light waves after they have passed through all the optical gates, but it also became apparent that the new arrangement has advantages of its own: it offers the designer of an optical switching matrix a wider choice in terms of the arrangement of the intermediate paths between the gates, and therefore reduces either the optical power losses on those paths or the noise at the output of the selector, at no additional cost.